.htm File — HyperText Markup Language

HTM is the shortened file extension for HTML (HyperText Markup Language), originating from the three-character extension limitation in MS-DOS and early Windows systems. HTM and HTML files are completely identical in format, content, and browser rendering — the only difference is the extension length. Both are served with the text/html MIME type and processed by the same HTML parser in web browsers. The .htm extension was common on Windows-based web servers (IIS) and tools throughout the 1990s and 2000s, while Unix-based systems typically used .html. Modern web development overwhelmingly favors the .html extension, but .htm files are still encountered in legacy websites, older content management systems, and Windows-generated web content. Web servers, browsers, and development tools treat both extensions identically. When migrating legacy .htm files to modern systems, no conversion is needed — renaming to .html is purely cosmetic. Both extensions support all HTML features including HTML5, embedded CSS and JavaScript, forms, multimedia, and semantic markup. Search engines index .htm and .html pages equivalently.